The present invention relates to a phase shifter or a phase equalizer, which is used to provide flat group delay time when it is used in a filter circuit which has sharp amplitude characteristics. The present phase equalizer is, for instance, used in an intermediate frequency amplifier stage in an FM radio receiver or an AM radio receiver.
In an intermediate frequency stage of a radio receiver set, a ceramic filter has been used for providing sharp amplitude (or attenuation response) characteristics to get rid of interference from an adjacent frequency radio station.
FIG. 1 shows a prior ceramic filter for tuning with sharp amplitude characteristics. In the figure, the numeral 1 is a ceramic substrate, 2 and 3 are a pair of separated electrodes attached on one surface of the substrate, 4 is a common electrode attached on the other surface of the substrate.
FIG. 2 shows a circuit diagram of a filter circuit which has two ceramic filters of FIG. 1 with the capacitor C for coupling the filters.
By the way, an intermediate frequency (IF) filter is required to have sharp amplitude characteristics so that no interference from an adjacent frequency station occurs. However, it should be noted that a filter with sharp amplitude response has in general non-flat delay characteristics. When the delay characteristics are not flat, the demodulated signal has a significant amount of distortion. So, when the signal is speech or music, the quality is substantially deteriorated, and when the signal is data, the error rate increases.
If we try to obtain flat delay characteristics in a prior art, the amplitude response of a filter is deteriorated, and we suffer from interference.
FIGS. 3 through 5 show delay characteristics and attenuation characteristics of a prior ceramic filter. In those figures, the curves A.sub.1, A.sub.2 and A.sub.3 show group delay characteristics, B.sub.1, B.sub.2 and B.sub.3 show attenuation response, the vertical axis at right side shows delay time (.mu.S), the vertical axis at left side shows attenuation (dB), and the horizontal axis shows frequency.
FIG. 3 is the case that the value Q of the ceramic filter is 300, FIG. 4 is the case that Q is 200, and FIG. 5 is the case that Q is 100.
It should be noted in FIG. 3 which has high value of Q (Q=300) that the attenuation response curve B.sub.1 is sharp, and the selectivity of a receiver set is excellent. However, the group delay characteristics A.sub.1 has two peaks with deep concaved recesses between the peaks. This group delay characteristics would provide much distortion to the demodulated signal.
On the other hand, FIG. 5 has relatively flat delay characteristics A.sub.3. However, it has broad attenuation response, which is not satisfactory in selectivity. Further, FIG. 5 has large insertion loss for a filter (the insertion loss is about 9 dB). Therefore, if we try to couple a plurality of filters of FIG. 5 in a series to obtain satisfactory attenuation response, the total insertion loss would increase in proportion to the number of filters, and therefore, that attempt is not practical.
The characteristics of FIG. 4 are intermediate between those of FIG. 3 and FIG. 5, however, in case of FIG. 4, both the attenuation response and the delay characteristics are unsatisfactory.
There have been attempts to equalize or compensate undesirable phase characteristics by an IF filter. Some of them are an equalizer using an LC circuit, and an equalizer using an interdigital filter. However, prior equalizers have the disadvantage that the size is too large to mount it in a small housing of a miniaturized radio set.
Other prior arts are Japanese patent laid open publications Nos. 136256/79 and 136257/79, which show a phase equalizer with a single ceramic energy confine type resonator. However, they have the disadvantages that the design for obtaining the desired characteristics is critical, and that it is difficult to obtain the desired bandwidth and the desired phase characteristics.